1. Field of the Invention.
The present invention relates to transmission shifting mechanisms for small transmissions such as used in riding lawn mowers and the like. More specifically, the field of the invention is that of small transmissions having a detent arrangement for biasing the shifting mechanism.
2. Prior Art.
One transmission having an in-line shifting mechanism is disclosed in U.S. Pat. No. 4,966,574, assigned to the assignee of the present invention, the disclosure of which is explicitly incorporated by reference. In this arrangement, the input shaft turns a bevel gear which is freely rotatable on the shift shaft. The bevel gear engages a spur gear on the counter shaft. The other counter shaft spur gears engage speed change spur gears which are freely rotatable on the shift shaft. The shift fork of the shifting mechanism moves the keys and thereby engages one of the speed change spur gears on the shift shaft, with the keys being rotatably fixed to the shift shaft to couple the counter shaft and the shift shaft. Another gear on the shift shaft engages the ring gear of the differential to transfer the rotational motion of the shift shaft to the output shafts, which are driven by a speed determined according to the gear ratio.
Such a transmission may include a shift arm biasing mechanism like that disclosed in U.S. Pat. No. RE32,125, assigned to the assignee of the present invention, the disclosure of which is explicitly incorporated by reference. In the disclosed arrangement, the shift fork of the shifting mechanism includes a detent plate with a peripheral edge having a series of detent seats. The detent seats have also been provided in the side of the detent plate. A spring and ball are disposed in the housing of the transmission, positioned to be adjacent to the detent seats and arranged so that the ball is biased by the spring into one of the detent seats. The shift arm is connected to a manually operable control and provides sufficient "feel" so that the vehicle operator positively senses the shift positions.
In order to provide the detent arrangement of the above mentioned U.S. Pat. No. RE32,125, a separate detent plate is secured to the shift fork by welding, brazing, or similar method. The detent plate must have detent seats formed at predetermined positions, and must be large enough to provide detent seats for all the selectable gears. The biasing mechanism is provided by forming an aperture in the housing, and assembling the mechanism so that the spring biased ball engages the detent seats. Providing such a detent plate and biasing mechanism adds to the expense of the transmission, both in terms of additional materials and manufacturing steps. The additional materials includes the detent plate itself and the biasing mechanism. The additional manufacturing steps include forming the detent seats in each detent plate, and assembling the biasing mechanism in the housing aperture.
What is needed is a shifting mechanism detent arrangement which eliminates some of the conventional components and thereby lowers the cost.
Another need is for a shifting mechanism which requires fewer manufacturing steps.
Also needed is such a shifting mechanism which provides sufficient "feel" for positively sensing the shift position.